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Re: attempts to reconciliate quote-semantics and "context"-semantics (Was: Re: RDF dataset semantics again)

From: Ivan Herman <ivan@w3.org>
Date: Thu, 23 Aug 2012 15:11:18 +0200
Cc: RDF WG <public-rdf-wg@w3.org>
Message-Id: <BBEB8470-DD31-4D19-B602-34E23A169488@w3.org>
To: Antoine Zimmermann <antoine.zimmermann@emse.fr>
Hi Antoine,

(Sorry, this mail is a little bit long. tl;dr: I actually like the direction of where this is going:-)

I'll try to summarize the different pieces of the puzzle in your proposal in one place, and in somewhat laymen's term. I do that to check my own understanding but also to make it easier for the group to follow; this may be helpful so that we could make an informed decision. I hope you don't mind me doing that. (I will also have technical questions on some details... see below,)

1st piece.

We already have a series of 'Entailment Regimes' scattered around the various documents: Simple, RDFS, OWL-RL, RIF... These are already reused in the SPARQL Entailment regime document (and all have a URI to identify them, b.t.w.). We would add one more, namely the no-semantics one [1]. As far as I can see the major difference between this 'No-Semantics' and the 'Simple' is that 'No-Semantics' ignores all the intricacies of blank nodes and is just a mapping for RDF graphs in terms of a model.

2nd piece

What Richard has defined a while ago in [2] for datasets is, actually, a kind of a parametrized interpretation of datasets. If I take a (G, <n1,G1>, <n2,G2>,...,<nk,Gk>), the parameters are the choices of the entailment regimes, that may be different for each Gi, as well as for the default graph. This means that, in theory, the application/data provider can choose the combination of entailment regimes per graphs: G1 would be considered for OWL-RL, whereas Gk is subject to RIF, for example.

As a base line, ie, if no other information is known, then the No-Semantics is the entailment regime used for all the G1,G2,...,Gk graphs. (Does this also apply to the default graph?)

3rd piece

The 'extension' point which, until now, relied on typing in our earlier brainstormings, is performed through a separate set of data description statements (as bit VoiD-like, if I may say so). To copy from your page:

@prefix  ds:  <datasetInferenceVocabulary> .
[ a  ds:DefaultGraphInferenceRegime ;
  ds:onGraphIRI  "http://www.example.com/"^^xsd:anyURI ;
  ds:withRegime  <http://www.w3.org/ns/entailment/RDFS> . ]
[ a  ds:GraphInferenceRegime ;
  ds:onGraphIRI  "http://www.example.com/"^^xsd:anyURI ;
  ds:withRegime  <http://www.w3.org/ns/entailment/Simple> . ]

would specify the parameters for the interpretation per [2].

As you say, these statements may be part of the Default graph of a dataset, but may also be external, and an inference engine would then use those.

First of all, is this a fair summary of what you propose?

_Personally_, I like where this is going, although I am not sure it would cover all the use cases we had in the past. For example, one of the use case that did come up is what we used to call 'merge semantics', meaning that all inferences and constraints checks are made on the merge of all the graphs (including the default graph). This is not covered by this structure but, maybe, this is not important.

(I am also a little bit worried about the complexity for users. We shall see the feedbacks.)

Also, some technical issues/questions:

1. (I should have looked at that yesterday, sorry.) I found the terminology used in [2] a bit confusing and I have the feeling that there is an overload of the letter 'E'... At present, it says

[[[
Moreover, dataset interpretations are defined with respect to an entailment regime E, as defined in SPARQL 1.1 Entailment Regimes. Let KE be the set of all E-interpretations. The interpretation of an RDF Dataset (G, (<n1>,Gn1), ..., (<nk>,Gnk)) over vocabulary V is a pair (I,Con) where I is an E-interpretation of G (the default graph) and Con is a mapping from V to KE.

A dataset-interpretation (I,Con) of a vocabulary V wrt entailment regime E satisfies an RDF Dataset (G, (<n1>,Gn1), ..., (<nk>,Gnk)) iff I E-satisfies G, and for all i in [1..k], Con(ni) exists and E-satisfies Gni.
]]]

I *could* read this sentence as asking that *all* constituent graphs, including the default graph, must use the same interpretation. Isn't it:

[[[
Moreover, dataset interpretations are defined with respect to an entailment regime E, as defined in SPARQL 1.1 Entailment Regimes. Let KE be the set of all interpretations as defined in SPARQL 1.1 plus the No-Semantics. The interpretation of an RDF Dataset (G, (<n1>,Gn1), ..., (<nk>,Gnk)) over vocabulary V is a pair (I,Con) where I is an E-interpretation of G (the default graph), and Con is a mapping from V to KE.

A dataset-interpretation (I,Con) of a vocabulary V wrt entailment regime E-satisfies an RDF Dataset (G, (<n1>,Gn1), ..., (<nk>,Gnk)) iff I E-satisfies G, and for all i in [1..k], Con(ni) exists and Con(ni)-satisfies Gni.
]]]

(Note that last 'Con(ni)-satisfies Gni'.)

Terminologically and in line with the outline you had, I actually o.k. to call that an 'E-interpretation of the dataset'; after all, each entailment regime is parametrized, wether it applies to the default graph or not.

2. If I choose to use, say, OWL as an entailment regime for Gi, how do I specify *which* OWL Ontology should be used for that purpose? I know this is also a question one may about current graphs, too, but the situation becomes a bit more complex if we have many different graphs. Would we require some sort of a follow-your-nose on the terms used in Gi and allow the system to find the relevant ontologies? Or would we suppose that, at least conceptually, Gi has all the ontologies as part of the graph already, and we decide that this is not our problem?

For RIF, SPARQL/RIF introduced the rif:usedWithProfile. Maybe something similar, but also for OWL, should be added to those parameters that you defined.

3. My other issue is a question. The No-semantics means that there are no semantic conditions on the graphs. However, looking at first table of semantic conditions in section 1.4 of the RDF Semantics document, although that section says 'denotation of ground graphs', the first four condition is not dependent at all on blank nodes, it just defines a bit what a graph structure is. I believe that can be safely added to the No-semantics as a semantic condition. I *think* I understand why the other two are not added as conditions: we do not want to get into the blank node store at this point.

Whether we expand the no-semantics with those or not is not *really* important, though; I just wanted to understand why those were left out.

Thanks

Ivan


[1] http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs/Dataset-semantics-2.0
[2] http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs/Dataset-semantics


On Aug 22, 2012, at 16:57 , Antoine Zimmermann wrote:

> So, I made a new wiki page in a tentative to reconciliate the different
> semantics.
> 
> http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs/Dataset-semantics-2.0
> 
> Basically, I define a super-weak semantics of RDF graph, which can be
> used as an underlying entailment regime for the dataset semantics of [1].
> 
> Then, I put an example of a possible vocabulary to allow more expressiveness. The vocabulary is mirroring some of the terms of SPARQL 1.1 service descriptions [2].
> 
> This truly makes the "base" semantics very very weak but allows one to extend it to any variant on top of it.
> 
> 
> [1] http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs/Dataset-semantics
> [2] http://www.w3.org/TR/2009/WD-sparql11-service-description-20091022/
> 
> 
> AZ
> 
> Le 22/08/2012 16:33, Ivan Herman a écrit :
>> 
>> On Aug 22, 2012, at 15:54 , Pat Hayes wrote:
>> 
>>> 
>>> On Aug 22, 2012, at 2:04 AM, Ivan Herman wrote:
>>> 
>>>> 
>>>> On Aug 21, 2012, at 21:48 , Pat Hayes wrote: [snip]
>>>>>> 
>>>>>> 
>>>>>> Antoine, I have the impression that we are actually in
>>>>>> agreement. The document we have put forward has two essential
>>>>>> points:
>>>>>> 
>>>>>> - we would have a default semantics in the form of the
>>>>>> quoting semantics
>>>>> 
>>>>> 
>>>>> Whoa. I do not know what y'all mean by a "default semantics".
>>>>> Is this a default that can be overridden? If so, I know of NO
>>>>> semantic theory  anywhere in logic or linguistics that can
>>>>> provide this. If y'all want this, you are on your own.
>>>>> 
>>>>> If not, what exactly is it supposed to mean?
>>>>> 
>>>>> Pat
>>>> 
>>>> 
>>>> What I meant is: this is the semantics that is standardized to be
>>>> valid in the absence of any other indication. I did not say
>>>> anything else.
>>> 
>>> And Antoine agrees. OK, then a better term would be "weak" or
>>> "minimal" semantics. "default" sounds like nomonotonicity (being
>>> overridable) to me.
>> 
>> Agreed, sorry for my sloppiness. 'Minimal' sounds indeed good to me.
>> 
>> Ivan
>> 
>> 
>>> 
>>> Pat
>>> 
>>>> 
>>>> Ivan
>>>> 
>>>> 
>>>> ---- Ivan Herman, W3C Semantic Web Activity Lead Home:
>>>> http://www.w3.org/People/Ivan/ mobile: +31-641044153 FOAF:
>>>> http://www.ivan-herman.net/foaf.rdf
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>> 
>>> ------------------------------------------------------------ IHMC
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>>> 
>>> 
>>> 
>>> 
>> 
>> 
>> ---- Ivan Herman, W3C Semantic Web Activity Lead Home:
>> http://www.w3.org/People/Ivan/ mobile: +31-641044153 FOAF:
>> http://www.ivan-herman.net/foaf.rdf
>> 
>> 
>> 
>> 
>> 
>> 
> 
> -- 
> Antoine Zimmermann
> ISCOD / LSTI - Institut Henri Fayol
> École Nationale Supérieure des Mines de Saint-Étienne
> 158 cours Fauriel
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> France
> Tél:+33(0)4 77 42 66 03
> Fax:+33(0)4 77 42 66 66
> http://zimmer.aprilfoolsreview.com/
> 


----
Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
FOAF: http://www.ivan-herman.net/foaf.rdf
Received on Thursday, 23 August 2012 13:11:46 UTC

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